Lenses utilizing the phenomenon of refraction of light are made from transparent optical material. As a result, the wavelength region for which these lenses can be used is limited by the optical characteristics of the optical material from which they are made. Moreover, these lenses can only be used over wavelength regions in which light can be transmitted, and even in these regions, chromatic aberration occurs due to dispersion arising from wavelength characteristics of refractive index.
On the contrary, a mirror utilizing the phenomenon of reflection of light can be used over a wide wavelength region without having the above chromatic aberration. Reflective surface made of metals such as aluminum, silver, and gold have high reflectance over a wide wavelength region from ultraviolet to infrared.
On the other hand, when a single mirror is used, aberration can be reduced over a very narrow range near a particular point (for instance, a point where optical axis and image plane intersect), however, at points away from the point for example, aberration (for instance, asymmetrical aberration such as coma aberration and astigmatism) become greater. Therefore, although light incoming from a particular direction can be condensed, light incoming from an object of a certain size, that is, an incident light having a certain angle range cannot form an image.
At least two mirrors are required to capture an optical image of an object. As the number of mirrors increases there are more chances of generation errors in production or assembly of an image capturing apparatus. Therefore, increasing the number of mirrors is not practical. Therefore, it is preferable that the image capturing apparatus is provided with the minimum required number of mirrors, i.e. two. However, increasing the number of mirrors increases design freedom, and can improve the image formation performance over a broader angle range.
The examples of conventional image capturing apparatus with two mirrors are Cassegrain-type, Ritchey-Chretien-type, and Gregorian-type image capturing apparatuses as mentioned in the book “Hansha-bo-enkyo (reflection-type telescopes)”, written by Yamashita, Yasumasa (University of Tokyo Press, 1992, p. 115). FIG. 7 represents the structures of the Cassegrain-type and Ritchey-Chretien-type image capturing apparatuses.
For example, when FIG. 7 represents the Cassegrain-type image capturing apparatus, the numeral 101 represents a parabolic primary mirror (with an aperture in this example), and the numeral 102 represents a hyperbolic secondary mirror (with an axis of revolution identical to that of the primary mirror 101). The numeral 103 indicates a photodetector positioned near the object image formed by the primary mirror 101 and the secondary mirror 102.
In the Cassegrain-type image capturing apparatus, the primary mirror 101 and the secondary mirror 102 are rotationally symmetrical in structure enabling the production using a turning lathe. The forms and the placement of the primary mirror 101 and the secondary mirror 102 are determined so that under the condition in which spherical aberration can be eliminated, other aberration can be reduced in well balance. The photodetector 103 is, for example, a film, a CCD or the like which converts light intensity distribution into electric signals.
On the other hand, when FIG. 7 represents the Ritchey-Chretien-type image capturing apparatus, the numeral 101 represents a hyperbolic primary mirror, the numeral 102 represents a hyperbolic secondary mirror, and the numeral 103 indicates a photodetector placed near the object image formed by the primary mirror 101 and the secondary mirror 102.
In the Ritchey-Chretien-type image capturing apparatus, axis of revolution of the secondary mirror 102 is identical to that of the primary mirror 101, like in the Cassegrain-type image capturing apparatus. The forms and the placement of the primary mirror 101 and the secondary mirror 102 are determined so that under the condition in which spherical aberration and coma aberration of aberration can be eliminated, other aberration can be decreased in well balance.
FIG. 8 shows the structure of a Gregorian-type image capturing apparatus. In FIG. 8, the numeral 111 represents a parabolic primary mirror (with an aperture herein), the numeral 112 represents an ellipsoidal secondary mirror (wherein its axis of revolution is identical to that of the primary mirror 111), and the numeral 113 indicates a photodetector placed near the object image formed by the primary mirror 111 and the secondary mirror 112.
In the Gregorian-type image capturing apparatus, the light incoming from the object first forms an image after it reflects from primary mirror 111, and then is focused into an image again by the secondary mirror 112. The forms and the placement of the primary mirror 111 and the secondary mirror 112 are determined so that under the condition in which spherical aberration can be eliminated, other aberration can be reduced in well balance, like in the Cassegrain-type image capturing apparatus.
However, the conventional image capturing apparatuses with two mirrors mentioned in the above reference can only reduce aberration over a very narrow field of view and the angle range of the object is very small as it is obvious from the fact that these are image capturing apparatuses both used as telescopes. In other words, when image capturing is to be done over a wide field of view, the periphery of the image becomes blurred. Therefore, a clear image cannot be obtained.
Moreover, in the conventional image capturing apparatuses with two mirrors, the light from the object is shielded by the secondary mirror before it reaches the primary mirror, undergoing a problem known as vignetting. Therefore there is a loss of luminous energy and the image becomes blurred due to diffraction.
In the light of the above, it is an object of this invention to provide an image capturing apparatus with two mirrors in which a clear image can be obtained without vignetting even when an image of the object is captured over a wide field of view.